Lymphocytes recognize antigen and each other through idiotypes. The complementary recognition between lymphocytes creates a self-maintaining regulatory network. In this application I discuss why regulatory cellular interactions between idotypic and antiidiotypic cells can best be studied in a well defined antigenic system. Phosphorycholine (PC) antigens induce idiotypically homogeneous T and B cell responses which can be studied in vivo and in vitro. My research attempts to characterize idiotype-specific interactions within and between T-B compartments and the role of macrophages in the selection of idiotypes by PC-specific T and B lymphocytes.